Transgene expression in liver samples collected at 8 and 32 weeks after treatment with dual vectors for gene targeting as neonates or adults. Chimeric m-hFIX mRNA levels in liver measured by reverse-transcription qPCR using primers/probe spanning the junction of murine and hFIX cDNA. Liver samples were collected from targeting vector–treated FIX-KO mice at 8 weeks after vector treatment via two-thirds partial hepatectomy and at 32 weeks during necropsy. Untargeted control mice were euthanized at 8 and 32 weeks after vector treatment for liver collection. Each circle represents an individual mouse. Data are mean ± SD. A 2-tailed Mann-Whitney U test was used to compare m-hFIX mRNA levels at weeks 8 and 32. n.s., not statistically significant.